Human immunodeficiency virus (HIV), which is also called human T-lymphotropic virus type III (HTLV-III), lymphadenopathy-associated virus (LAV) or AIDS-associated retrovirus (ARV), was first isolated in 1982 and has been identified as the etiologic agent of the acquired immunodeficiency syndrome (AIDS) and related diseases. Since then, chemotherapy of AIDS has been one of the most challenging scientific endeavors. So far, AZT, ddC, ddI, 3TC and D4T have been approved by FDA and are being clinically used as drugs for the treatment of AIDS and AIDS-related complex. Although these FDA-approved drugs can extend the life of AIDS patients and improve their quality of life, none of these drugs are capable of curing the disease. Bone-marrow toxicity and other side effects as well as the emergence of drug-resistant viral strains limit the long-term use of these agents..sup.1 On the other hand, the number of AIDS patients worldwide has increased dramatically within the past decade and estimates of the reported cases in the very near future also continue to rise dramatically. It is therefore apparent that there is a great need for other promising drugs having improved selectivity and activity to combat AIDS..sup.1 Several approaches including chemical synthesis, natural products screening, and biotechnology have been utilized to identify compounds targeting different stages of HIV replication for therapeutic intervention..sup.2
Very recently, the screening program at the National Cancer Institute has discovered a class of remarkably effective anti-HIV natural products, named calanolides, from the rain forest tree Calophyllum lanigerum, with calanolide A, 1, being the most potent compound in the reported series..sup.3 For example, calanolide A demonstrated 100% protection against the cytopathic effects of HIV-1, one of two distinct types of HIV, down to a concentration of 0.1 .mu.M. This agent also halted HIV-1 replication in human T-lymphoblastic cells (CEM-SS) (EC.sub.50 =0.1 .mu.M/IC.sub.50 =20 .mu.M)..sup.3 More interestingly and importantly, calanolide A was found to be active against both the AZT-resistant G-9106 strain of HIV as well as the pyridinone-resistant A17 virus..sup.3 Thus, the calanolides, known as HIV-1 specific reverse transcriptase inhibitors, represent novel anti-HIV chemotherapeutic agents for drug development.
A natural source of calanolide A is limited..sup.4 Consequently, a practical synthesis of the natural product must be developed for further study and development to be carried out on this active and promising series of compounds. Herein, we describe, a method for the synthesis of (.+-.)-calanolide A, (+)-calanolide A and calanolide A analogues. ##STR1##